(i) Field of the Invention
The present invention relates to a radio terminal station apparatus for an SDH network, particularly to a method of selecting an operation clock in a radio terminal station apparatus for an SDH communication connected to a wired transmission line or a wireless transmission line.
(ii) Description of the Related Art
SDH (Synchronous Digital Hierarchy) is a standard established for standardizing synchronous networks, and its details are regulated by ITU-T or the like. FIG. 6 is a view showing the structure of an STM-1 frame in an SDH frame structure. The STM-1 frame consists of an SOH (Section Over Head) consisting of a frame synchronization, a parity of a transmission line, a transmission line for maintenance data and so on, an AU pointer for indicating the head of a data signal sequence when changing clocks, a payload that is a data signal sequence, and so on.
Besides, an S1 byte in the SOH is defined for transmitting a quality that is the quality of a clock in a synchronous network. The quality indicated by the S1 byte is defined by ITU-TG.707 or the like. By extracting the quality Q from the S1 byte in the SOH, the quality of the clock of a transmission line can be known. FIG. 7 shows an example of the contents of this quality Q.
Besides, in the SDH network, synchronization of the whole of the network is necessary for keeping the quality of the network high, and the clock source that is the origin of the synchronous network becomes important. For this reason, there is a case that, in addition to a primary clock supply source of the highest quality, a secondary clock source is provided for use when a trouble arises on this primary clock source and it can not be used, and the primary and secondary clock sources are switched over with the quality in the S1 byte. At this time, for the secondary clock, a clock source of a lower quality than the primary clock is mostly employed for an economical reason or the like.
Further, each NE (Network Element: synchronous terminal station apparatus) mostly has an internal clock source that is used when either of the primary and secondary clock sources can not be used. Besides, in apparatus for transmitting data signal sequences including SDH frames, there are transmission terminal station apparatus that make communications through wired transmission lines such as optical cables and radio terminal station apparatus that are connected to such transmission terminal station apparatus and make communications through wireless transmission lines.
FIG. 8 shows an example in which an SDH network is built with such transmission and radio terminal station apparatus. In FIG. 8, NE1, NE2, NE7 and NE10 denote transmission terminal station apparatus, and NE3, NE4, NE5, NE6, NE8 and NE9 do radio terminal station apparatus. U1 denotes a primary clock source that is the origin of an ordinary SDH synchronous network, and U2 does a secondary clock source.
Besides, it is the example in which the network construction branches into two directions at the wired transmission lines from NE4 to NE5 and NE8. Further, each of SYS1 and 2 has a set of up-link or down-link data signal sequences including SDH frames, and MUX denotes a multiplexer, and DMR does a digital microwave radio equipment. As understood from FIG. 8, in a wireless transmission line, data signal sequences of plural system are transmitted and received through a single antenna in general, so the network never branches at the wireless line.
By the way, although it was above described that, when the SDH network is constructed, the whole of the network is synchronized, and a case that a trouble arises on the clock of the origin of synchronization, or the like is dealt with by switching over, at this time, if opposed apparatus are synchronized with line clocks from opposite directions, respectively, a loop is generated in the clock synchronization construction, as a result, the network synchronization construction breaks. This operation is called timing loop. For preventing this operation, in the S1 byte indicating the clock quality, as shown in FIG. 7, xe2x80x9cDon""t use for syncxe2x80x9d of Q=F is defined, and the line clock of the route on which Q=F is detected is not used as the clock in the apparatus.
A conventional radio or transmission terminal station apparatus has clock selection means in which a process of a flowchart shown in FIG. 9A or 9B is carried out as a method of determining a quality to insert. FIG. 9A shows a flowchart used when the transmission directions of the whole system are the same like radio terminal station apparatus. In this flowchart, when the clock in the apparatus is generated from a line clock, Q=F on the qualities to multiplex on all systems in which the direction of multiplexing the qualities is opposite to the line clock that was the origin of the clock in the apparatus.
FIG. 9B shows a flowchart used in a transmission terminal station apparatus or the like, wherein, in case that the clock in the apparatus is generated from a line clock, Q=F is multiplexed only when the direction of multiplexing the qualities is opposite to the line clock that was the origin of the clock in the apparatus and the system is the same route.
Next, operations of the SDH network constructed with conventional radio terminal station apparatus having the process flowchart of FIG. 9A will be described with reference to FIGS. 10A and 10B. In FIGS. 10A and 10B, FIG. 10A shows synchronization of clocks in a stationary state. In this FIG. 10A, line clocks of both routes of (NE1xe2x86x92NE2xe2x86x92NE3xe2x86x92NE4) and (NE1xe2x86x92NE2xe2x86x92NE5xe2x86x92NE6) all are synchronized with the output clock of U1. Here, (NE1xe2x86x92NE2) denotes direction of clock signal transmitted from NE1 to NE2. All of multiplexed qualities of routes of (NE4xe2x86x92NE3xe2x86x92NE2xe2x86x92NE1) and (NE6÷NE5xe2x86x92NE2xe2x86x92NE1), which are in the opposite directions of the above routes, are Q=F. P denotes a priority showing the degree of priority, and shows information on the degree of priority assigned to each clock of the input port part of SYS1 and 2 or the like of each apparatus, respectively.
Here, as shown in FIG. 10B, supposing a case that a trouble or the like arises on the output clock of U1 and it can not be used, all line clocks of route of (NE4xe2x86x92NE3xe2x86x92SYS1 of NE2xe2x86x92NE1) are synchronized with the secondary clock of U2. However, in NE2, because the line clock from NE3 becomes the clock in the apparatus, the quality sent out from SYS2 of NE2 to NE5 becomes the quality of Q=F for inhibiting a timing loop. Thereupon, in NE5, because the quality from the SYS2 of NE2 is Q=F, it can not be used as the clock in the apparatus and it operates with the internal clock of the highest quality in the apparatus. Accordingly, NE6 is also synchronized with the internal clock of NE5, and NE5 and NE6 operate separately from the conventional synchronous network.
Successively, operations of the SDH network constructed with conventional radio terminal station apparatus having the process flowchart of FIG. 9A and conventional transmission terminal station apparatus having the process flowchart of FIG. 9B will be described with reference to FIGS. 11A to 11D. FIG. 11A shows synchronization of clocks in a stationary state. In FIG. 11A, all line clocks of route of (NE1xe2x86x92NE2xe2x86x92NE3) are synchronized with the output clock of U1, and all multiplexed qualities of route of (NE3xe2x86x92NE2xe2x86x92NE1), which are in the opposite direction of the above route, are Q=F.
In this state, like FIG. 11B, supposing a case that a trouble arises on the output of U1, because the clock having the highest quality in NE1 is the internal clock, line clocks of route (NE1xe2x86x92NE2xe2x86x92NE3) are once synchronized with the internal clock of NE1. Thereupon, in NE3, because the clock having the highest quality changes from the line clock from NE2 to the secondary clock of U2, switchover is done in the order of (NE3xe2x86x92NE2xe2x86x92NE1) so as to synchronize with the output clock of U2, as shown in FIG. 11C.
At this time, though multiplexing qualities in the radio terminal station apparatus of NE2 to the direction of NE3 is Q=F at both of SYS1 and SYS2, in the transmission terminal station apparatus of NE1, as shown in the flowchart of FIG. 9B, to other than the system in which the line clock that was the origin of the clock in the apparatus was extracted, because the quality of the clock in the apparatus is multiplexed, as shown in FIG. 11C, Q=3 is sent out from SYS2 of NE1 to NE2.
Thereupon, in NE2, because the quality from NE3 and the quality from NE1 become the same and the clock from NE1 that has a higher priority is selected as the clock in the apparatus, immediately after the selection, as shown in FIG. 11D, the multiplexed quality of route (NE2xe2x86x92NE1) is converted into Q=F. Accordingly, the line clock from NE2 can not be selected in NE1, as a result, because of returning to the state of FIG. 11B, the clock in the apparatus becomes in a timing loop state.
For preventing this, necessary is a change of establishment such as that, as for the line clock of route of (NE1xe2x86x92NE2), in order not to take redundant constructions at SYS1 and SYS2, the quality of SYS2 of NE2 is made to be always Q=F.
In this manner, in conventional radio terminal station apparatus, in relation to processing on the qualities to multiplex, there are a problem that the synchronous network construction breaks in case that a route of the network is branched at wired transmission lines, a problem that a redundant construction for a route having a line clock of a high quality can not be taken, furthermore, a point at issue that there is a possibility to generate a timing loop.
The present invention has been made in view of the above points at issue and aims to provide a radio terminal station apparatus for an SDH network, wherein, by defining a special priority value in a system beforehand, a redundant construction for a route having a line clock of a high quality is maintained and the synchronous network construction never breaks even in case that a wired transmission line is branched, and a method of selecting an operation clock thereof.
In a radio terminal station apparatus for an SDH network of the present invention, the quality/clock extracted from each system and each direction is input to a clock selection part. In this clock selection part, first, in a quality judgement circuit, the route information and quality value of the clock of the highest quality from among the qualities of an external clock, an internal clock and a line clock from each system are output to a control signal generation circuit. In the control signal generation circuit, in case of a single clock of the highest quality in the input quality information, the clock is selected, and, in case of a plurality of clocks, the route of the highest priority judged by a priority judgement circuit is selected, and a clock selection signal for using it as the clock in the apparatus, and the quality and extraction route information of the clock to be the clock in the apparatus are output.
In a multiplexing quality generation circuit, the direction and system to multiplex the quality are compared with the input direction and system of the clock in the apparatus, and, only in case of not the same system but the opposite direction and when the priority establishment of the input opposite to the insertion direction is a special value defined in advance, the same quality as the clock in the apparatus is multiplexed.
As described above, when determining a quality to multiplex, by using the priority established on the input opposite to the direction to multiplex the quality, a communication terminal station apparatus can be provided wherein, even in case of the transmission route of the data signal sequence branching, the synchronous network of the branched route never breaks, and further, in case that there are a plurality of routes having clocks of high qualities in the same direction, the degree of redundant can be kept and the synchronous network construction never breaks.